What happens in the atmosphere of a tidally locked world in the habitable zone of a red dwarf? We have solid work suggesting through simulations that habitable conditions could exist there, but it’s also true that we’re in the early stages of these investigations and we have no actual examples to work with. Drawing hasty conclusions is always dangerous, particularly when we’re talking about the details of atmospheric circulation on a planet no one has ever seen.
Take Gliese 581g. Assuming it exists — and there is still a bit of doubt about this, although the consensus seems to be that it’s really there — we can place it in a temperature zone that would allow life. We don’t know for a fact, though, that it isn’t a water world, covered entirely with deep ocean, a planet that migrated from beyond the snowline into its present position. And even if it is a rocky planet with a substantial atmosphere, our simulations of atmospheric circulation only represent the best that is known today. This early in the game, we should expect surprises.
Addendum: I’m evidently wrong about the consensus re Gliese 581g, as it appears that its existence really is doubtful. Check the comments to this post to learn more.
A Lesson from a ‘Hot Jupiter’
An instructive case is the planet Upsilon Andromedae b, which although hardly in a habitable region (it’s a hot Jupiter orbiting — and tidally locked to — an F-class primary in a 4.6-day orbit), has yielded useful information about conditions within its own atmosphere. The blindingly obvious notion that a tidally locked planet should have its hottest region directly in the center of the Sun-facing side turns out to be untrue, and not just by a small amount. For new work via the Spitzer Space Telescope tells us that Upsilon Andromedae b’s hot spot is fully 80 degrees away from high noon, on the side rather than the star-facing center.
Ian Crossfield, lead author of the paper on this work, has this to say about Upsilon Andromedae b:
“We really didn’t expect to find a hot spot with such a large offset. It’s clear that we understand even less about the atmospheric energetics of hot Jupiters than we thought we did.”
Upsilon Andromedae b is thus a cautionary tale, as if more were needed, about the dangers of over-extrapolation. Exoplanet atmospheric science is, in any case, an infant discipline, and aside from the simulations of red dwarf planet atmospheres (Joshi, Haberle and Reynolds’ work at NASA Ames kicked this off in a 1997 paper in Icarus), we’ve focused almost entirely on hot Jupiters, using transits where available to study atmospheric composition. We’ve found water, methane, carbon dioxide and carbon monoxide in their atmospheres in a series of remarkable investigations.
The Spitzer instrument measured the total light of Upsilon Andromedae b and its star in the infrared, which is how the unusual temperature distribution came to light. The system turns out to be brightest when the planet is at the side of the star as seen from Earth rather than when it is behind the star, showing its Sun-facing side (Upsilon Andromedae b does not transit). It’s incumbent upon theorists to come up with a solution to this latest atmospheric riddle. In the hunt are star-planet magnetic interactions and the effects of supersonic winds but one suspects we’ll be hearing of other possibilities.
The Danger of Quick Assumptions
Back to the red dwarf question. We have to be careful about making too many assumptions about Gliese 581g and any other planet discovered to be in the habitable zone of a red dwarf. I admit to being optimistic, but there is so much we don’t yet know. Suppose that Gl 581g is indeed an ocean planet. Centauri Dreams reader Dave Moore has commented in the past about a paper by Timothy Merlis and Tapio Schneider (Caltech) that offers encouraging results re tidally locked ocean planets, with moderate temperatures and a super-rotating atmosphere that maintains a mild climate (interestingly, the coldest parts of the planet are the poles, not the anti-stellar point).
Is a rocky world in the habitable zone going to be equally moderate in its temperatures, and will its precipitation patterns follow those suggested by Merlis and Schneider for a water world? We can speculate all we want about the hydrological cycle and so on, but it’s the encounter between theory (and simulations) with observational data that handily supplies the monkey wrench, as it just has in the case of Upsilon Andromedae b. Until we have observations of tidally locked red dwarf planets beyond their wraith-like appearance in hundreds of hours of radial velocity data, continued caution on habitability is the only recourse.
The Merlis and Schneider paper is “Atmospheric dynamics of Earth-like tidally locked aquaplanets,” in press at the Journal of Advances in Modeling Earth Systems (full text). The Upsilon Andromedae b paper is Crossfield et al., “A New 24 micron Phase Curve for Upsilon Andromedae b,” accepted by The Astrophysical Journal (preprint).